Spectrums used in an existing wireless communications system are classified into two types: a licensed spectrum (English: licensed spectrum) and an unlicensed spectrum (English: unlicensed spectrum). Any operator may deploy a device in an unlicensed frequency band, for example, a Wireless Fidelity (full spelling in English: wireless fidelity, WiFi for short) device in a 2.4 GHz and 5 GHz frequency band.
In the prior art, on an unlicensed carrier, all data frames are transmitted in a time contention manner, and sending of a data frame is not started until a time resource is obtained through contention. If some devices can always obtain a time resource through contention, some other devices can never obtain a time resource through contention, and therefore can never occupy an unlicensed carrier. Therefore, fairness of occupying an unlicensed carrier by multiple devices cannot be ensured in the prior art.